The field is two-stroke cycle internal combustion engines. Particularly, the field relates to ported, uniflow-scavenged, two-stroke cycle engines with exhaust gas recirculation. More particularly, the field includes two-stroke cycle engines with one or more ported cylinders and uniflow scavenging in which an exhaust gas recirculation (EGR) construction provides a portion of the exhaust gasses produced by the engine in previous cycles for mixture with incoming charge air to control the production of NOx during combustion.
A two-stroke cycle engine is an internal combustion engine that completes a power cycle with a single complete rotation of a crankshaft and two strokes of a piston connected to the crankshaft. One example of a two-stroke cycle engine is an opposed-piston engine in which a pair of pistons is disposed in opposition in the bore of a cylinder for reciprocating movement in opposing directions. The cylinder has inlet and exhaust ports that are spaced longitudinally so as to be disposed near respective ends of the cylinder. The opposed pistons control the ports, opening respective ports as they move to their bottom center (BC) locations, and closing the ports as they move toward their top center (TC) locations. One of the ports provides passage of the products of combustion out of the bore, the other serves to admit charge air into the bore; these are respectively termed the “exhaust” and “intake” ports.
In FIG. 1, a two-stroke cycle internal combustion engine 49 is embodied by an opposed-piston engine having at least one ported cylinder 50. For example, the engine may have one ported cylinder, two ported cylinders, three ported cylinders, or four or more ported cylinders. Each cylinder 50 has a bore 52 and exhaust and intake ports 54 and 56 formed or machined in respective ends thereof. The exhaust and intake ports 54 and 56 each include one or more circumferential arrays of openings in which adjacent openings are separated by a solid bridge. In some descriptions, each opening is referred to as a “port”; however, the construction of a circumferential array of such “ports” is no different than the port constructions shown in FIG. 1. Exhaust and intake pistons 60 and 62 are slidably disposed in the bore 52 with their end surfaces 61 and 63 opposing one another. The exhaust pistons 60 are coupled to a crankshaft 71, the intake pistons are coupled to the crankshaft 72.
When the pistons 60 and 62 of a cylinder 50 are at or near their TC positions, a combustion chamber is defined in the bore 52 between the end surfaces 61 and 63 of the pistons. Fuel is injected directly into the combustion chamber through at least one fuel injector nozzle 100 positioned in an opening through the sidewall of a cylinder 50.
With further reference to FIG. 1, the engine 49 includes an air management system 51 that manages the transport of charge air provided to, and exhaust gas produced by, the engine 49. A representative air management system construction includes a charge air subsystem and an exhaust subsystem. In the air management system 51, the charge air subsystem includes a charge air source that receives intake air and processes it into charge air, a charge air channel coupled to the charge air source through which charge air is transported to the at least one intake port of the engine, and at least one air cooler in the charge air channel that is coupled to receive and cool the charge air (or a mixture of gasses including charge air) before delivery to the intake port or ports of the engine. Such a cooler can comprise an air-to-liquid and/or an air-to-air device, or another cooling device. The exhaust subsystem includes an exhaust channel that transports exhaust products from exhaust ports of the engine to an exhaust pipe.
With reference to FIG. 1, the air management system 51 includes a turbocharger 120 with a turbine 121 and a compressor that rotate on a common shaft 123. The turbine 121 is coupled to the exhaust subsystem and the compressor 122 is coupled to the charge air subsystem. The turbocharger 120 extracts energy from exhaust gas that exits the exhaust ports 54 and flows into the exhaust channel 124 directly from the exhaust ports 54, or from an exhaust manifold 125. In this regard, the turbine 121 is rotated by exhaust gas passing through it. This rotates the compressor 122, causing it to generate charge air by compressing intake air. In some instances, the charge air subsystem includes a supercharger 110; in these instances, the charge air output by the compressor 122 flows through a charge air channel 126 to a cooler 127, whence it is pumped by the supercharger 110 to the intake ports. Air compressed by the supercharger 110 can be output through a cooler 129 to an intake manifold 130. The intake ports 56 receive charge air pumped by the supercharger 110, through the intake manifold 130. Preferably, but not necessarily, in multi-cylinder opposed-piston engines, the intake manifold 130 is constituted of an intake plenum that communicates with the intake ports 56 of all cylinders 50.
The air management construction shown in FIG. 1 is equipped to reduce NOx emissions produced by combustion by recirculating exhaust gas through the ported cylinders of the engine. The recirculated exhaust gas is mixed with charge air to lower peak combustion temperatures, which lowers NOx emissions. This process is referred to as exhaust gas recirculation (“EGR”). The EGR construction shown in FIG. 1 utilizes exhaust gasses transported via an EGR loop external to the cylinder into the incoming stream of fresh intake air in the charge air subsystem. The recirculated gas flows through a conduit 131 under the control of the valve 138.
EGR constructions for uniflow-scavenged two-stroke cycle opposed-piston engines require a positive pressure differential from the intake manifold to the exhaust manifold in order to scavenge the cylinders during their port open periods. Thus, the pressure in the intake port of a cylinder must always be greater than in the exhaust port in order for exhaust gas to flow through the EGR channel into the charge air subsystem. In instances illustrated by FIG. 1, a supercharger in the charge air channel provides this positive pressure. However, there are other instances in which a turbo-charged opposed-piston engine may not include a supercharger. In such cases, there is a need to ensure positive flow of recirculated exhaust gasses for effective EGR operation.